Articular surfaces and ligaments
The wrist joint is formed by the concave distal radius and articular disc above and the convex proximal carpal row below; the ulna itself does not touch the carpus.
- Distal radius: its articular surface has a lateral triangular facet for the scaphoid and a medial quadrilateral facet for the lunate, separated by a low ridge. The radial styloid projects further distally than the ulnar styloid. On the back, Lister's tubercle (the dorsal tubercle) separates the second and third extensor compartments.
- Distal ulna: the head articulates with the ulnar notch of the radius at the distal radio-ulnar joint. The ulnar styloid projects from its back.
- Triangular fibrocartilage complex (TFCC): the articular disc runs from the medial edge of the radius to the base of the ulnar styloid. It separates the distal radio-ulnar joint from the wrist joint, lies between the ulnar head and the triquetrum, and is the main stabiliser of the distal radio-ulnar joint.
The capsule is strengthened by palmar and dorsal radiocarpal ligaments, which make the hand follow the radius in pronation and supination, and by weak collateral ligaments. The midcarpal joint, between the two carpal rows, adds a substantial share of flexion and extension.
Extensor compartments
The extensor retinaculum, a band of deep fascia across the back of the wrist, is tethered to the radius and ulna by septa that make six compartments, each with its own synovial sheath.
| Compartment | Tendons | Clinical point |
|---|---|---|
| 1 | Abductor pollicis longus, extensor pollicis brevis | De Quervain's tenosynovitis |
| 2 | Extensor carpi radialis longus and brevis | Intersection syndrome where compartment 1 crosses it |
| 3 | Extensor pollicis longus | Turns round Lister's tubercle; ruptures after distal radius fracture |
| 4 | Extensor digitorum, extensor indicis | Posterior interosseous nerve lies in its floor |
| 5 | Extensor digiti minimi | Lies over the distal radio-ulnar joint |
| 6 | Extensor carpi ulnaris | Groove beside the ulnar styloid; tendon can sublux |
All of these tendons are supplied by the radial nerve: extensor carpi radialis longus by the main trunk above the elbow, and the rest by the deep branch and posterior interosseous nerve.
Carpal landmarks and the flexor retinaculum
The front of the wrist is defined by four bony landmarks that anchor the flexor retinaculum and turn the carpal arch into the carpal tunnel.
- Laterally: the tubercle of the scaphoid and the ridge (tubercle) of the trapezium. The scaphoid tubercle is felt at the base of the thenar eminence at the distal wrist crease.
- Medially: the pisiform and the hook of the hamate. The pisiform is felt at the base of the hypothenar eminence, with the hook of the hamate distal and lateral to it.
The retinaculum carries the median nerve and nine long flexor tendons beneath it. Flexor carpi radialis passes through a separate groove in the trapezium, within the lateral attachment. The ulnar nerve and artery pass over the retinaculum in Guyon's canal, and palmaris longus and the palmar cutaneous branch of the median nerve cross in front of it.
Blood supply and innervation
The wrist is supplied by the palmar and dorsal carpal arches, formed by carpal branches of the radial and ulnar arteries with contributions from the anterior and posterior interosseous arteries. Articular branches come from the anterior interosseous nerve (median), the posterior interosseous nerve (radial) and the dorsal and deep branches of the ulnar nerve. The terminal posterior interosseous nerve is sometimes divided during wrist surgery to reduce pain.
Clinical relevance
Wrist problems are identified by locating tenderness over a specific compartment or landmark.
- Distal radius fracture: the commonest fracture of the upper limb. In the Colles' pattern the distal fragment is displaced and angled dorsally, giving a dinner-fork deformity; the Smith's pattern is displaced towards the palm. Acute carpal tunnel syndrome can follow.
- Extensor pollicis longus rupture: weeks after a fracture, even an undisplaced one, the tendon frays at Lister's tubercle and the patient cannot lift the thumb off a table.
- De Quervain's tenosynovitis: pain over the radial styloid, reproduced by Finkelstein's test (ulnar deviation with the thumb held in the palm).
- TFCC tear: ulnar-sided wrist pain and clicking, often with instability of the distal radio-ulnar joint.
- Ganglion: a cystic swelling, most often on the dorsum over the scapholunate ligament.
- Scaphoid fracture: tenderness in the anatomical snuffbox or over the scaphoid tubercle.
On the specimen
The wrist station combines the articular end of a dry radius and ulna with the extensor tendons on the back of a dissected wrist.
- Radius: the surface for the scaphoid is the lateral, triangular facet running on to the styloid; the surface for the lunate is the medial, squarer facet next to the ulnar notch.
- Ulnar styloid: the short peg on the back of the ulnar head; do not confuse it with the radial styloid, which is larger and more distal.
- Front of the carpus: the scaphoid tubercle and the ridge of the trapezium lie laterally; the ridge is the vertical crest on the trapezium's palmar surface.
- Tendons, radial to ulnar: abductor pollicis longus and extensor pollicis brevis together at the thumb; extensor carpi radialis longus and brevis to the second and third metacarpals; extensor pollicis longus running obliquely round Lister's tubercle; extensor digitorum with extensor indicis deep to it; extensor digiti minimi; extensor carpi ulnaris beside the ulnar styloid.